1. Field of the Invention
The invention relates to a method for transducer self monitoring.
II. Description of Related Art
Transducers or sensors are used increasingly extent to control and regulate physical variables in modern measuring and automatization applications. These transducers convert quantities such as pressure or temperature into an electrical signal, for example, that subsequent electronics process further. The demand for transducers and subsequent signal processing systems has created a need for the transducer to detect its own malfunctioning so as to reduce its effect. The transducer can, for example, malfunction by changing its sensitivity or zero point. An important source of error, however, can come at the coupling of the transducer to the physical variable. Therefore, monitoring the entire measuring chain from the transducer to the subsequent signal processing system is more effective when done closer to the beginning of the measuring chain.
From DE-AS 2,437,438, for example, is known a method for increasing the accuracy of an analog measuring processes. At least two precisely known calibration values are applied in succession at the input of an analog measuring chain. The measuring chain connects the inputs toa digital system where they are stored in digital form. The transmission properties of the entire analog measuring chain can be determined using these calibration values received at the input of the transducer of the measuring chain. These values preferentially have the form of the physical variable to be measured. This permits the precise assignment of defined physical values to the digital measuring signals stored in the measuring process. At least two calibration values are applied at the input of the measuring chain. One calibration value preferably corresponds to the non-loaded state and the other to the end scale deflection of the transducer. In addition, provisions are made for storing several calibration values to aid in detecting errors in linearity.
However, this method requires physically disconnecting the measuring chain at the input of the transducer. The disconnection interrupts the coupling of the variable to the input of the transducer. As a result, a physically equivalent calibration value must be applied at the input of the transducer rather than as the physical variable to be measured.
A physically disconnected measuring chain is, for example, known from "VDI-Bericht No. 566, Automatische Messysteme, VDI Verlag, Dusseldorf, 1985". On pages 361 and 362 of this article is disclosed an inductive relative path sensor which is supplemented by a calibration addition. The magnetic core of the inductive sensor is supported on axis in the probe tip so as to be displacable. During a measurement the magnetic core is normally fixed relative to the probe tip by a permanent magnet. The magnetic coil is excited for calibration and the magnetic core is pulled to a stop on axis. The known displacement path supplies a jump signal to the measuring chain which indicates the transmission factor in mV/mm as long as the transducer has a linear response.
However, the physical separation of the measuring chain caused by uncoupling the transducer from the physical variable to be measured makes for a more difficult measurement. In addition, the coupling of the tranducer to the physical variable cannot be monitored during the calibration process because the actual physical variable to be measured cannot be accessed.
R. K. Chohan and F. Abdullah disclose in Transducer Tempcom Conf. Papers, tavistock 1983, pages 1 to 10, a method for self monitoring a temperature sensor in which the coupling of the sensor to the physical variable to be measured is maintained during the monitoring process. Briefly increasing the current through the temperature sensor to a multiple of the normal value causes the temperature sensor to be locally heated so that the dynamic response of the measured signal conveys information regarding the sensor and its thermal coupling to the medium being measured. Quantitative determination of the sensitivity of the sensor is, however, not possible with this method because the response to the measuring signal over time is such that the information regarding the sensitivity of the sensor cannot be separated from the information regarding the thermal contact of the sensor with the surrounding medium.